Twig allows expressions everywhere. These work very similar to regular
PHP and even if you're not working with PHP you should feel
comfortable with it.

And if you test in pure PHP the following expression :

$var = true;
if ($var == "top") {
echo 'ok';
}

It will also display ok.

The point is : you should not compare variables of different types. Here, you compare a bool with a string : if your string is not empty or if it does not contains only zeros, it will evaluate to true.

You can also have a look to the PHP manual to see how comparison are made with different types.

Edit

You can use the sameas test to make strict comparisions, and avoid type juggling matters.

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